An integrated circuit, otherwise known as a "chip" or a "die," is typically supported and protected by a microcircuit package. Electrical input and output signals and power to the chip are available through metallic, electrically conductive areas known as the bond pads located around a perimeter of the die. For access to the chip circuitry, proper electrical interconnections are required between these bond pads and a plurality of metallic connector leads on a lead frame of the microcircuit package to ensure accurate signal communication between the chip and external circuits. Various types of microcircuit packages are known, such as single in-line package (SIP), dual in-line package (DIP), pin grid array (PGA), quad flat package (QFP), and leadles chip carrier (LCC). In these examples, the lead frame is housed in a ceramic or a plastic package prior to electrical coupling of the package to a printed circuit board. However, other microcircuit packages may have the chip directly coupled to the printed circuit board or mounted on a substrate, typically along with other chips, which is then coupled to the printed circuit board to affectuate the electrical coupling of the chip to the printed circuit board. The process of forming the electrical interconnections between the bond pads and the leads of the microcircuit package is more commonly known as "chip-connection," or "chip interconnect," with the most commonly used chip interconnect technology in the microelectronics industry being wirebonding.
FIG. 1 shows one example of forming chip interconnections by wirebonding. Typically, after die 10 is bonded to micrcocircuit package 50 (for example, with die-bonding epoxies), a wirebonder is then used to form bonds 35-36 to couple electrically conductive wire 40 between bond pad 20 and connector lead 30. Since bonds 35-36 form an essential signal path for transmitting and receiving electrical signals between die 10 and connector lead 30, it is essential that the wirebonder used to form these bonds produces high quality and accurate bonds to ensure proper function and signal communication between chip 10 and other external circuits, and also to ensure long term reliable operation of chip 10.
Several wirebonding techniques are known in forming metallic bonds 35-36. The three most common wirebonding techniques are ultrasonic (U/S) bonding, thermocompression (T/C) bonding, and thermosonic (T/S) bonding. Prior art wirebonders typically incorporate one of these known techniques in the formation of bonds 35-36. For example, in U/S bonding, the wirebonder guides wire 40 to a bonding site, such as bond pad 20, and then wire 40 is pressed onto the bond site by bond tool 60 (See FIGS. 2a-2c), such as a capillary. While capillary 60 presses wire 40 to the bond site, a burst of ultrasonic energy is applied to capillary 60. The combination of the pressure and the vibration of capillary 60 against metallic wire 40 and the metallic surface of the bond site results in the formation of a metallurgical cold weld between wire 40 and the bond site.
In thermocompression bonding, or T/C bonding, the bond surface of capillary 60 is heated typically to 300.degree.-400.degree. C. as the capillary is pressed against a bond site. As shown in FIG. 2b, a ball 42 is formed by Electronic Flame Off (EFO) device 45 prior to bonding. The elevated temperature of capillary 60 causes the metal at the bond site to deform easily under the pressure applied by capillary 60, thus facilitating the formation of a metallic bond.
FIGS. 2a-2c illustrate thermosonic bonding, or T/S bonding, whereby the principles of U/S bonding and T/C bonding are combined. In T/S bonding, ultrasonic power 45 is apply to capillary 60 to augment the formation of metallic bond 35 as capillary 60 is held against the bond site. The application of ultrasonic power 45 to capillary 60 allows bonding to be accomplished at a lower bond surface temperature than is required in T/C bonding.
Regardless of which wirebonding technique is implemented in forming bonds 35-36, a common problem of prior art wirebonders is the inability to accurately and quickly detect the amount of bond force being applied by the bond tool to the bond surface. Prior art wirebonders typically use mechanical indirect sensing means to sense when the capillary touches the bond pad, and to sense the bond force applied.
FIG. 3a is an example of a prior art wirebonder. In wirebonder 65, bond tool 60, such as a capillary, is attached to transducer 80. Typically, bond wire 40 is threaded through capillary 60 as a convenient way to supply bond wire 40 to wirebonder 65. Transducer support frame 71 is coupled to transducer 80 to allow a joined rigid controlled movement of transducer 80 together with support frame 71. Support frame 71 also allows movement separate to transducer 80. Connecting rod 74 couples support frame 71 to motor crank 72, while motor crank 72 is coupled to a z-motion actuator (not shown), such as a z-motor. As connecting rod 74 moves vertically, bond force spring 73 attached to support frame 71 applies an associated force against transducer 80 to move capillary 60 towards and against bond site 160, thereby applying bond force to bond wire 40 and bond site 160.
Alternatively, a voice coil is used for providing vertical movement directly to the transducer, without the need for spring 73, as is know, for example, in the K & S 1484 machine.
Contact sensor 78 is provided to detect when capillary 60 first contacts bond site 160. Contact sensor 78 is also coupled to a central processing unit (CPU), not shown, of wirebonder 65 to provide a signal indicating to the CPU when contact is detected between capillary 60 and bond site 160. Once contact is detected, the CPU then directs motor crank 72 to move connecting rod 74 vertically one or more motor increments until the vertical distance travelled by connecting rod 74 corresponds to a predetermined vertical distance value stored in the CPU. The bond force value applied by capillary 60 on bond site 160 is thus approximately calculated from the vertical distance travelled by connecting rod 74. Therefore, the subsequent distance travelled by connecting rod 74 following contact of capillary 60 with bond site 160 roughly corresponds to the application of a predetermined bond force by capillary 60 on bond site 160. Damper solenoid 102 is provided to adjust the damping associated with movements of transducer 80 and capillary 60 on bond site 160.
FIG. 3b shows a simplified representation illustrating how the dynamic bond force on bond site 160 can be determined. Referring to FIG. 3b, bond force f.sub.b may be expressed as EQU f.sub.b =f(0)+f(x); where (1)
f(x)=Kx, with K equivalent to the spring constant, and x equivalent to the distance travelled by the spring; and
f(0)=the initial force required to cause deflection of the spring (for example, 35 grams).
FIG. 3c illustrates a typical dynamic bond force response of the force exerted by capillary 60 on bond site 160. On initial impact of capillary 60 with bond site 160, undesirable oscillation occurs which may cause damage such as cratering or puncturing of a bond pad. It is therefore desirable to minimize or eliminate the initial oscillation caused by capillary 60 on bond site 160 upon impact.
In another prior art wirebonder, the K&S 1484 manufactured by Kulicke and Soffa of Willow Grove, Pa., a motor provides the direct drive of the bond tool as it is lowered vertically, i.e. along a z-axis, down to the bond pad. A current sensor senses the motor current which is applied to the driving device, and compares the sensed current with an acceptable predefined current value. A sensed current which is greater than the predefined value indicates that the capillary has either made contact with the bond pad, or the capillary is exerting bond force against the bond pad. An increased sensed current corresponds to an increased bond force being applied to the bond pad. Thus, the K&S 1484 wirebonder indirectly and approximately detects the bond force by evaluating the applied motor current.
Prior art wirebonders provide an indirect means of determining bond force which results in inaccurate and slow detection and response time to excessive bond force being applied to the bond pad. Excessive bond force which is not timely sensed and remedied by the wirebonder often results in "cratering", a deformation of the chip's bond pad. Excessive force of the bond tool on the bond pad may also deform the silicon layers beneath the pad. Cratering results not just in bond pad deformation, but it may also seriously damage chip circuitry and operation. Other problems may also result from exerting excessive bond force on the die, such as puncturing the die to create a void, thus resulting in an ineffectually formed bond which is susceptible to unbonding, or decoupling of the bond wire to bond pad.
Therefore, there is a need for a wirebonder that provides real-time feedback sensing and controlling of the bond force exerted on the bond pad, without detracting from the quality and reliability of the electrical connection produced.